The goal of this project is to define the mechanisms by which T-cell surface molecules function in the recognition of foreign cell surface molecules. A large panel of DP-specific cytotoxic T-cell (CTL) clones has been developed to facilitate the study of membrane molecules that are involved in T-cell recognition and triggering. These CTL clones have been used to analyze the roles of the T3, T4, T8, and LFA-1 surface molecules in T-cell recognition of the class II major histocompatibility complex (MHC) antigens. The results indicate that the role of the T4 molecule may be to facilitate T-cell recognition of class II molecules by reacting with a nonpolymorphic region of the molecule and thereby increasing the overall tightness of binding of T-cells to target cells. The T3 molecule appears to be involved in an affinity-dependent triggering function and is not involved in target cell binding. Studies have also been conducted on the molecular requirments for CTL recognition of target cells using the technique of DNA-mediated gene transfer. The gene which encodes the heavy chain of an HLA-A3 molecule has been transfected into murine L cells. The HLA-A3 molecule is expressed at the surface of the transfected cells and these transfectants are susceptible to lysis by HLA-A3-specific CTL. Antibody blocking studies indicate that the T8 and LFA-1 molecules are functionally involved in recognition of the transfected cells. These results demonstrate that the only human gene product on the target cells that is required for HLA-A3-specific CTL recognition is the HLA-A3 heavy chain, and that the target molecules of the putative cell interaction molecules T8 and LFA-1 are either on the HLA class I heavy chain or on murine molecules that are highly homologous to their human counterparts.